1. Field of the Invention
The present invention relates to an antenna device for a compact wireless equipment, and more particularly, to an antenna device which is incorporated in an earphone connected to a compact wireless equipment.
2. Description of the Related Art
In the field of compact wireless equipments, importance is placed on improvements on portability which can be accomplished by reducing the size and weight of the equipment.
A portable FM radio may be used with an earphone which incorporates an antenna. The use of an earphone with a built-in antenna contributes to improved portability of FM radio because an antenna need not be additionally provided.
In an antenna built-in earphone for use with a portable FM radio, a cable of the entire earphone from an earphone jack is utilized as an antenna. The earphone for use with a compact wireless equipment such as a portable FM radio typically has a cable of one to two meters long. Therefore, the antenna incorporated in the earphone can readily achieve a desired impedance and gain in a frequency band with long wavelengths such as the VHF band (the wavelength of which is on the order of ten to one meter).
An earphone may be used with a mobile telephone in some cases, as disclosed in JP-04-200047-A where such an earphone incorporates an antenna. A mobile telephone described in JP-04-200047-A has an antenna disposed in a head unit, thereby enabling the user to set the mobile telephone at an arbitrary place when the earphone is used. Also, the mobile telephone described in JP-04-200047-A has two antennas in the head unit such that one of the antennas can be selected by a switch for connection to the mobile telephone through a connection cord. This switching is intended to acquire a sufficient field strength by selecting an antenna which presents a higher field strength.
However, the conventional art techniques described above cause the following problems.
The portable FM radio may be used with the earphone, the cable of which may be partially wound up, or partially placed in a pocket. Such a use would result in variations in impedance and gain of the antenna incorporated in the earphone, possibly failing to sufficiently manifest the effect of the antenna.
On the other hand, in the mobile telephone described in JP-04-200047-A, one of the antennas disposed in the head unit is connected to the mobile telephone through the switch and connection cord. In view of high frequency characteristics, the entirety from the antenna in the head unit to the connection cord appears to be continuous, and functions as an antenna. Therefore, the antenna would experience variations in impedance and degraded characteristics if it is used with the wound-up connection cord or if it is placed in a pocket or a bag in its use.